diff --git a/c3l_membership/views.py b/c3l_membership/views.py
--- a/c3l_membership/views.py
+++ b/c3l_membership/views.py
@@ -53,6 +53,25 @@ def index():
 
     if form.validate_on_submit():
 
+        today = date.today()
+        if form.birthday.data:
+            born = form.birthday.data
+            try:
+                birthday = born.replace(year=today.year)
+            except ValueError:
+                birthday = born.replace(year=today.year, month=born.month + 1, day=1)
+
+            age = today.year - born.year
+
+            if birthday > today:
+                age -= 1
+
+            if age < 18:
+                form.minor_member.data = True
+                form.student.data = True
+            else:
+                form.minor_member.data = False
+
         if (
             form.minor_member.data
             or form.student.data
@@ -90,7 +109,6 @@ def index():
         elif form.payment.data in ("digicash", "satispay"):
             price = price * 100
 
-        now = date.today()
         xml_data = {
             "status": status,
             "name": form.fullname.data,
@@ -113,14 +131,14 @@ def index():
                 ),
             ),
             "voting": 1 if form.membership.data == "regular" else 0,
-            "date": now,
+            "date": today,
         }
 
         html = render_template(
             "member.html",
             form=form,
             price=price,
-            year=now.year,
+            year=today.year,
             voting=True if form.membership.data == "regular" else False,
             xml=xml_template.format(**xml_data),
         )